It is generally known to determine the location of a fault in an electrical conductor of a test object by emitting an electrical voltage pulse from an electrical pulse voltage source, and transmitting that pulse into the test object, i.e. the electrical conductor such as an electrical line or a cable. If there is a fault in the conductor, such as a complete or partial break, this causes a local change in the characteristic impedance of the conductor being tested or monitored. This particular impedance step will lead to a partial reflection of the transmitted voltage pulse depending on the severity of the impedance change. In case of a complete break of the conductor, the voltage pulse will be totally reflected at the location of the fault. If the fault is characterized as only partial, then the extent of how much of the incident voltage pulse is reflected depends on the extent of change in impedance the fault poses to the cable. If the change of impedance at the location of the fault is too small to be detected, then an additional high voltage source can be optionally applied at the input of the electrical conductor so as to charge the cable being tested in order to surpass the breakthrough voltage of the intermittent or partial fault and thereby achieve a controlled spark-over or arcing at the fault location. The low impedance of the arc causes a total reflection of the incident pulse at the location of the former undetectable intermittent fault, hence reflecting the incident voltage pulse completely. The optional high voltage source for fault ignition is further combined with extra testing equipment including a pulse echo meter or time domain impulse reflectometer as well as a measurement value detection circuit for detecting and evaluating one or more echos or reflected pulses that are received by the pulse echo meter at the input of the electrical conductor. The reflection is recorded by the time domain impulse reflectometer, in order to measure the time difference between the point of time at which the voltage pulse is emitted and the instant at which the reflected pulse arrives at the input of the electrical conductor. Then the distance from the input end of the electrical conductor to the location of the fault is determined based on the time difference measured before and the propagation velocity of the cable. This distance provides the location of the fault in the conductor.
Typically, the pulse echo meter or measuring device is integrated with the other equipment on a testing cart or instrument car, and is connected via a testing lead, e.g. a connecting cable, with the test object that is to be tested. The testing lead may have a length of up to 50 m, depending on the particular situation.
The characteristic wave impedance of the testing lead generally does not correspond to the input impedance of the test object, so that an additional interfering reflection of the input signal typically arises at the location of the connection interface of the testing lead to the test object. This additional interfering reflection appears in the pulse diagram of the reflection pulses received by the test equipment, but this interfering reflection provides no useful information for the user of the system, and also makes it more difficult to properly interpret or evaluate the measurements due to multiple reflections between this interfering reflection pulse and following impedance discontinuities.
Furthermore, the conventionally known equipment further includes a separation filter arranged in the propagation path of the pulse, e.g. connected between the test equipment and the test object. This separation filter serves to decouple the time domain measuring system and the pulse source from the optional high voltage source, yet to couple the pulses of the time domain measuring system into the test object. Due to its transfer function, this separation filter causes additional interference, or particularly a falsification of the pulse diagram, i.e. the train or sequence of pulses received back from the test object. In that regard, due to the natural self-resonance of the separation filter, a low frequency oscillation is superimposed on the pulse diagram. This makes it more difficult to determine the exact point of time of the base of a reflected pulse, so that an error arises in the transit time determination of the respective pulse.